Separation of titanium fluoride and niobium fluoride from gaseous uranium hexafluoride containing same

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF SELECTIVELY REMOVING TITANIUM OR NIOBIUM VALUES FROM A GASEOUS MIXTURE OF URANIUM HEXAFLUORIDE AND NIOBIUM PENTAFLUORIDE OR TITANIUM TETRAFLUORIDE BY PASSING THE MIXTURE THROUGH A BED OF PELLETIZED COMPLEX FLUORIDE AT A TEMPERATURE IN THE RANGE OF 200*F. TO 400*F.

United States Patent Olfice SEPARATION OF TITANIUM FLUORIDE AND ABSTRACT OF DISCLOSURE This invention relates to a method of selectively removing titanium or niobium values from a gaseous mixture of uranium hexafluoride and niobium pentafluoride or titanium tetrafluoride by passing the mixture through a bed of pelletized complex fluoride at a temperature in the range of 200 F. to 400 F.

BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

This invention relates generally to the processing of gaseous uranium hexafluoride and more particularly to a process for selectively removing titanium or niobium values from gaseous uranium hexafluoride containing titanium tetrafluoride or niobium pentafluoride.

In gaseous diffusion processes for the separation of uranium isotopes, uranium hexafluoride (UP is employed as the process gas. The UP commonly contains small amounts of various undesirable volatile metal fluorides. In UF derived from natural uranium the volatile fluorides may be corrosion products generated during the processing of the natural uranium to UF In UP derived from the processing of spent nuclear fuels containing irradiated uranium, the volatile fluorides may be corrosion products or they may be volatile-fluoride compounds of certain fission products.

For example, in fluid-bed volatility processes for recovering fissionable material from spent reactor fuels, volatile UF is produced by reaction of the fuel with gaseous fluorinating agents, such as ClF BrF or P In this reaction volatile fluorides of various corrosion products or fission products are produced-fluorides. such as TiF RuF SbF NbF and TaF Current Federal specifications on diffusion plant feed limit the permissible maximum concentration of titanium, ruthenium, antimony, niobium, and tantalum to one part-per-million each, based on the weight of the uranium in the feed.

Referring particularly to the removal of TiF and NbF5 from gaseous UF it is old in the art to remove NbF 3,625,661 Patented Dec. 7, 1971 Na SiF and Na ZrF will selectively sorb titanium tetrafluoride (TiF or niobium pentafluoride (NbF from streams of UF.; at a temperature in the range of 200 F. to 400 F. The sorbent can be prepared in the form of pellets by extruding or molding a mixture formed by blending water and a powder form of the complex fluoride. The pellets are dried, sintered, and fluorinated before use.

It is a general object of this invention to provide a method for selectively sorbing titanium and niobium values from a gaseous mixture of uranium hexafluoride from UF by passing the mixture through pelletized NaF. The NaF, however, must be maintained at comparatively high temperatures (e.g., temperatures exceeding 625 F.) to avoid excessive sorption of the valuable UF Operation at these high temperatures is both costly and inconvenient. Another disadvantage of NaF is that it is not an effective sorbent for Til- Pelletized MgF at a temperature of 250 F. has been used to sorb relatively large quantities of NbF and TiF from UF streams containing the same. But even at 250 F. the MgF sorbs quantities of UF which are significant, particularly when it is considered that a single diffusion plant trap may contain a thousand pounds of sorbent.

SUMMARY OF THE INVENTION This invention is based on the discovery that a porous bed of a particulate complex fluoride such as Na AlF and a volatile fluoride of titanium or niobium, the sorption process being characterized by comparatively low operating temperatures and by the removal of comparatively small amounts of uranium hexafluoride from the stream. Other objects of the invention will be made apparent in the following examples or in the appended claims.

' EXAMPLE I Removal of NbF with pelletized Na AlF Pellets of Na AlF were prepared by blending commercial-grade Na AlF (sodium aluminum fluoride) with water (10% by weight) and stearic acid (1% by weight). This mixture was extruded in the form of cylindrical pellets. After extrusion, the pellets were air-dried and then heated gradually to 1000 F. while being exposed to a stream of nitrogen containing a small percentage of fluorine. This treatment, which was conducted for three hours, sintered the pellets and fluorinated olf the stearic acid lubricant, leaving hardened pellets suitable for exposure to UF. The product pellets were silver-gray and measured approximately in diameter and A" in length. The pellets had a nitrogen-surface-area of 0.54 m. g. and a porosity of 0.368 (as measured by mercury intrusion).

A vertically oriented Monel cylinder was loaded with the abovementioned pellets to provide a porous bed. The cylinder had an internal diameter of three inches and was four feet long. It was provided with a gas inlet near its top end and with a gas outlet near its base. The cylinder was filled with pelletized Na AlF to a height of nearly four feet. For purposes of comparison, a perforated cup containing a single layer of MgF pellets was suspended in the cylinder above the Na AlF bed and four inches below the inlet. The MgF pellets had diameters ranging from A to 7 The following data relate to the performance of the sorbents when the gas feed to the above-mentioned cylinder comprised a mixture of N and UF., containing a known small concentration of niobium in the form of NbF Feed gas-1.0 lb./hr. N 24 lb./hr. UEF 75.6 p.p.m. Nb (on a uranium metal basis) average for entire run;

48 p.p.m. Nb for last 25 hours of run.

Pressure-17 p.s.i.a.

Velocity-0.5 ft./ sec.

Na AlF bed dimensions-diameter, 2%; depth, 3.8

Weight of Na AlF 15.0 lbs.

Bed temperature250 F.

Length of run-50 hours.

Niobium (Nb) in feed gas leaving Na AlF bed-Less than 0.5 p.p.m. Nb for last 25 hours of the run, indicating removal of better than 98% of the niobium.

Niobium (Nb) retention on Na AlF pellets-Niobium accounted for 1.638% of the final weight of the top 6 of the bed and for less than 0.01% of weight of the bottom 6" of the bed. (Bed was not saturated in 50 hours of operation.)

Uranium (U) retention on Na A1F pellets-Uranium accounted for 0.04% of the final weight of the top 6" 3 of the bed and for 0.15% of the weight of the bottom 6" of the bed.

Niobium (Nb) retention on MgF pellets-Niobium accounted for 3.32% of the final weight of the single layer of MgF pellets. (These pellets were fully saturated.)

Uranium (U) retention on MgF pelletsUranium accounted for 2.08% of the final weight of the MgF pellets.

In the foregoing run the amount of niobium in the feed averaged 75.6 p.p.m. It is apparent, therefore, that the Na AlF was a highly selective sorbent for removing NbF from the input stream. Note that the Na AlF exhibited a significantly lower degree of uranium loading than the MgF EXAMPLE II Sorption of TiF with pelletized Na AlF The following data relate to a TiF -sorption experiment conducted with Na AlF pellets of the kind described in Example I. In this test the pellets were contained in a perforated cup fitted into the upper end of the cylinder, just below the inlet. For comparison, a 6" layer of the above-mentioned MgF pellets was placed in the cylinder, just below the Na AlF pellets.

Feed gasl.0 lb./hr. N 23 lb./hr. UF 20 p.p.m. Ti

(on a uranium basis).

Pressurel7 p.s.i.a.

Velocity-O.5 ft./sec.

Na AlF bed dimensionsdiameter, 2%"; depth, /2".

Weight of Na AlF 40 grams.

Bed temperature250 F.

Length of run-50 hours.

Titanium (Ti) retention on Na AlF Titanium accounted for 1.12% of the final weight of the pellets.

Uranium (U) retention on Na AlF Uranium accounted for 0.31% of the final weight of the pellets.

Titanium (Ti) retention on MgF -Titanium accounted for 1.03% of the final weight of the MgF pellets.

Uranium (U) retention on MgF -Uranium accounted for 2.2% of the final weight of the MgF pellets.

In the foregoing run the amount of titanium in the feed was p.p.m. Thus, the Na AlF was a highly selective sorbent for removing TiF.,, from the input gas. As shown, the Na AlF exhibited significantly less uranium loading than the MgF The run just described was of an exploratory nature and was designed to establish whether Na AlF would in fact selectively remove titanium-fluoride without incurring high uranium loadings. This was substantiated by the results obtained. Given these results, one versed in the art can readily select the proper process parameters (bed dimensions, input velocity, etc.) to apply the process to a specific TiF -removal problem.

EXAMPLE III Removal of TiF with pelletized Na SiF Pellets of Na SiF were prepared by blending commercial-grade Na SiF with water (10% by weight) and stearic acid (1% by weight). This mixture was extruded in the form of cylindrical pellets. After extrusion the pellets were heated gradually to 1000 F. while being exposed to a stream of nitrogen containing a small percentage of fluorine. This treatment, which was conducted for three hours, dried the pellets and fiuorinated off the stearic acid lubricant, leaving hardened pellets suitable for exposure to UF The product pellets were whitishgray and measured approximately in diameter and /2" in length. They had a nitrogen-surface-area of 0.326 m. /g. and a porosity of 0.295 (as measured by mercury intrusion).

A perforated cup containing Na SiF pellets of the kind just described was fitted in the upper end of a vertically 4 oriented cylinder, having a gas inlet near its top and a gas outlet near its base. The cup was fitted in the cylinder just below its inlet. For comparison, a 6" layer of the above-mentioned MgF pellets was placed in the cylinder just below the Na SiF pellets.

The following data relate to the performance of the sorbents with a gas input to the cylinder comprising a mixture of N and UF containing a known small concentration of titanium in the form of TiF Feed gas1.0 lb./hr. N 23 lb./hr. UF 20 p.p.m. Ti,

based on uranium.

Pressurel7 p.s.i.a.

Velocity0.5 ft./ sec.

Na SiF bed dimensions-Diameter, 2%"; depth V2".

Weight of Na SiF 40 grams.

Bed temperature250 F.

Length of run--50 hours.

Titanium (Ti) retention on Na SiF Titanium accounted for 1.15% of the final weight of the pellets.

Uranium (U) retention on Na SiF Uranium accounted for 0.41% of the final weight of the pellets.

Titanium (Ti) retention on MgF Titanium accounted for 1.03% of the final weight of the MgF pellets.

Uranium (U) retention on MgF Uranium accounted for 2.2% of the final weight of the MgF pellets.

'In the foregoing run the amount of titanium in the feed was 20 p.p.m. It is apparent, therefore, that the Na SiF was a highly selective sorbent for Til- The uranium loading of the Na SiF was appreciably lower than that of the MgF EXAMPLE IV Sorption of NbF with pelletized Na SiF The following data relate to a run conducted in the same equipment and under the same conditions as described above, with the exceptions that the feed gas contained NbF rather than TiF and that a single layer of MgF pellets was provided in the inlet end of the cylinder, as in Example I. The niobium content of the feed gas was 75.6 p.p.m. Nb, based on uranium.

Niobium (Nb) retention on Na SiF -Analysis showed that 'Nb accounted for 2.79% of the final weight of the pellets.

Uranium (U) retention on Na SiF --Analysis showed that U accounted for 0.50% of the final weight of the pellets.

Niobium (Nb) retention of MgF -Niobium accounted for 3.3% of the final weight of the MgF Uranium (U) retention on MgF Uranium accounted for 2% of the final weight of the MgF As in the run made with TiF the Na SiF sorbent was found to be highly selective for the impurity of interest. Its uranium loading was less than that of the MgF EXAMPLE V Removal of TiF with pelletized Na ZrF Pellets of Na ZrF were prepared by extruding a blended mixture of commercial-grade Na ZrF water (10% by weight), and stearic acid (1% by weight). The extruded pellets were treated as described above in connection with Example III.

A perforated cup was filled with Na ZrF pellets prepared as described. The loaded cup was fitted in the upper end of a vertically oriented cylinder of the type referred to in Example IV. In this instance the feed gas comprised a mixture of N and UP containing a known small concentration of titanium as TiF The run data are summarized below. A 6" layer of MgF pellets of the size referred to in Example I was included in the cylinder below the Na ZrF pellets.

Feed gas-1 lb./hr. N 31.1 lb./hr. UF 1.5 lb./hr. F

22.5 p.p.m. Ti (on a uranium basis) Pressure17 p.s.i.a.

Velocity0.6 f.p.s.

Na ZrF bed dimensions i x 3" Weight of Na ZrF --30 grams Bed temperature-250 F.

Length of run-50 hours Titanium (Ti) retention on Na ZrF Titanium accounted for 0.43% of the final weight of the pellets.

Uranium (U) retention on Na ZrF Uranium accounted for 2.05% of the final weight of the pellets.

Titanium (Ti) retention on MgF -Titanium accounted for 0.81% of the final weight of the pellets.

Uranium (U) retention on MgF Uranium accounted for 3.05% of the final weight of the pellets.

In the exploratory run just described, the titanium loading for the l Ia ZrF was only about half that for the MgF However, the uranium loading was significantly less for the Na ZrF The small amounts of the complex fluoride sorbent used in Examples III, IV, and V were suflicient to establish selectivity for the impurity of interest and comparatively low uranium loading. Persons versed in the art can readily apply the process to specific TiF or NbF -sorption problems. It will be understood that the physical properties given above for the various pellets may not be the optimum. Suitable pellets have been prepared not only by extrusion but by forming in a die.

As mentioned, pelletized NaF has been used previously for the sorption of volatile metal fluoride impuritiese.g., NbF from UP gas streams, but it is subject to the disadvantages of high uranium loadings. For example, in runs conducted at 250 F, the UF loading of the NaF pellets was found to exceed 40%. Examples I-V, above, illustrate the unexpected finding that a complex fluoride containing sodium is less subject to such loading.

Although this method has been illustrated above in terms of the use of selected sodium-containing double fluorides as Sorption-agents, it will be apparent that it is within the scope of this invention to replace the sodium cation in these double fluorides with a cation of another alkali metal (lithium, potassium, rubidium, cesium) or oi the closely related alkaline-earth metals (beryllium, magnesium, calcium, strontium, barium).

It is likely that this method is applicable to the removal of other volatile metal fluoride impurities from gaseous UP -impurities such as TaF SbF and RuF The foregoing examples are illustrative and are not to be understood as limiting the scope of our invention, which is limited onl as indicated by the appended claims.

What is claimed is:

1. A method for selectively removing titanium and niobium values from a gaseous mixture of uranium hexafluoride and a gas selected from the group consisting of titanium fluoride and niobium fluoride comprising passing said mixture through a porous bed of a complex fluoride at a temperature in the range of 200 F. to 400 F., said complex fluoride comprising a cation of an alkali metal and a complex fluoro anion selected from the group consisting of AIF ZrF and SiF 2. The method according to claim 1 in Which the uranium hexafluoride gas issuing from the contacted bed contains less than 2 p.p.m. of either one of said values.

3. The method of claim 1 wherein said cation is a sodium ion.

References Cited UNITED STATES PATENTS 2,903,333 9/1959 Lowe et al. 23-337 2,952,511 9/1960 Maddock ot al. 23337 3,165,376 1/1965 Golliher 23337 3,178,258 4/1965 Cathers et al. 23337 3,423,190 1/1969 Steindler et al. 23326 3,458,291 7/1969 Riha et al. 23337 3,493,331 2/1970 Vancil et al. 2388 OTHER REFERENCES Smiley et 2111 :Removal of Impurities from Uranium Hexafluoride by Selective Sorption Techniques--Trans. Am. Nucl. Soc.vol. 10, #2, 1967, p. 507.

CARL D. QUARFORTH, Primary Examiner F. M. GITTES, Assistant Examiner U.S. c1. X.R 23-2 s, 21, 88, 326, as; 

